[DMN Designer] Do not default to a LiteralExpression when no expression is defined

Currently, the DMN Editor will default to a blank LiteralExpression on Save if the user did not provide an expression for an element.
However Error message is reported anyway to the user:

This also as the (imho undesired) side-effect that if the user was to re-open later that file, instead of a empty element, it would be a blank LiteralExpression

so the current behavior is not consistent across re-open of the editor.

Let's revert this default.

The DMN Editor on save should not default to a blank LiteralExpression if the user did not provide an expression for the element.
Once this change is applied from the f/e side, I am happy to be involved in order to assess which of the messages reported by the Validator or the Compiler are causing issue to the WB (if any).
Currently, the DMN Compiler will throw 1 Warning.
Currently, the DMN Validator will throw 1 Error (I can align this to be a Warn too).
Currently, the DMN Validator schema check is not reporting any XSD violation.

Manual acceptance test

Try to save default / empty

Business Central

• Decision node
• BKM node
• Cleared Function
• Context entry
• Invocation

Kogito

• Decision node
• BKM node
• Cleared Function
• Context entry
• Invocation